Coal liquefaction is a procedure discussed in the 1980 text Coal Liquefaction by Whitehurst and others. Several procedures are set forth which are believed to be representative of the art. The several procedures do not contemplate the procedure which is the subject matter of this disclosure.
This disclosure is directed to a process for coal liquefaction. Briefly, the process herein can be used with coal or lignite. The term "coal" will be used hereinafter to refer to coal or lignite, meaing a material that is rich in carbon. To be sure, there may be other constituents such as water subject to vaporization. There also may be ash content. Certain volatile components may be included and they are typical hydrocarbons which are driven off from the coal as it is heated.
In an earlier text Chemistry of Coal Utilization published by John Wiley in 1945, vacuum distillation of coal was discussed at pages 459-467. This method involves vacuum removal of oxygen and nitrogen followed by heating. In our process strict sense, the vacuum step is preliminary to hydrogen pressurization and heating.
This process contemplates the injection of hydrogen gas under pressure for conversion into hydrocarbons. The resultant hydrocarbon that is made is not a single compound but rather it is a spectrum of various hydrocarbons. For instance, the constituents may be in the paraffin series. However, other hydrocarbons can be formed including benzenes and aromatics. The conversion process herein taught has an initial step of grinding the particulate coal to particle sizes suitable to enable ready commingling with the hydrogen gas in a conversion chamber. Coal is introduced into a furnace to be mixed with hydrogen gas. However, an important preliminary step is the more or less total evacuation of atmospheric air from the vessel. In particular, this evacuates nitrogen and oxygen. Thereafter, the temperature is raised. As the temperature is raised and while it is sustained at the peak, various volatile constituents are driven off from coal including combustible hydrocarbons to be recovered. Water may also be driven off as the temperature is raised. More or less simultaneously with hydrogen injection at high pressure, the temperature is raised.
The method of the present invention teaches the use of a vacuum chamber around the reactor vessel. The vacuum chamber initially drops the pressure to a relatively low pressure, typically in the range of 10.sup.-2 torr. This is accomplished before the temperature is raised and before hydrogen is introduced. As the pressure in the vessel is lowered, various gases and vapors are boiled off and they also are removed by operation of the vacuum pump connected with the vacuum or pressure chamber. This continues during the reduced pressure step. The vessel is held at a reduced pressure for a period of time; then, both the hydrogen pressure and temperature are raised from the initial low pressure and room temperature until a final high temperature and pressure are achieved. It has been discovered that the final temperature should be in the vicinity of about 900.degree. F. and final pressure in the range of 1,000-2,000 psi.
Typically, hydrogen is introduced at a pressure steadily raised to about 2,000 psi. While this would ordinarily be a dangerous process, there is a preferred procedure of introducing the hydrogen gradually and ultimately raising the pressure to a maximum pressure. Conversion of the elemental carbon into various hydrocarbons is accomplished. After the maximum pressure of perhaps 1,000 to about 2,000 psi is accomplished, an elevated temperature is held for an interval to enable the conversion to complete. Optionally, stirring can be undertaken to enhance the contact of the particulate coal with the gaseous hydrogen. Fortunately, hydrogen in the elemental form is a relatively penetrating gas, and it becomes commingled intimately with the ground coal, thereby accomplishing a fairly rapid conversion of particulate coal into hydrocarbons. The process can be optionally enhanced by the incorporation of iron or iron ore as a catalyst. The use of such a catalyst appears to lower the maximum temperature required. Rather than typically operating at about 900.degree. F., the process can operate successfully at a lower temperaure, perhaps as low as 600.degree. F. at 1,000 psi. After an interval, the surplus hydrogen can be removed by vacuum pump and reclaimed for subsequent use. Some of the hydrocarbons are gaseous and must be separated from the surplus hydrogen. The pressure vessel can then be cooled to enable recovery of the coal, now in the form of liquefied or solid hydrocarbons. At room temperature some of the liquids will solidify into various tar like materials. The process is not specific in conversion, namely, providing a multitude of hydrocarbon compounds with various and sundry melt points and other physical characteristics.
It is believed that the referenced texts and other references in the area of coal liquefaction do not set out a procedure such as that discussed in summary fashion above. The present invention is therefore summarized as including a procedure or method as set forth above. One advantage of this procedure is that the discharge gases from the vessel are substantially captured and converted, yielding a process which is pollution free. To the extent that any volatile gases are liberated and removed during pressure reduction, gases can be converted. Later on, when the vessel is charged with hydrogen at elevated pressure and temperature, the gaseous hydrogen is commingled with the carbon (of the coal) for an interval, and is preferably thereafter removed for storage of surplus hydrogen.
Several advantages of the present procedure will be noted on review of the detailed description set forth below. Briefly, this invention relates primarily to a procedure or process for conversion of coal into liquid hydrocarbons accomplished by a reactor vessel within an evacuated pressure vessel.